WGO? Sonar X3x64: Master Bus Set To NONE But I Get Sound???

1st time for this one: I get sound when I shouldn't.
 
Behringer UFX1204USB interface has 2 outputs: 1/2 & 3/4.
If I set the Master bus output to NONE I get meter activity from MAINS 1/2 & sound from the interface 1/2 output.
 
If I set the Master bus output to 1/2 the sound increases slightly.
 
If I set the Master bus output to 3/4 I get meter activity from both MAINS & sound from both interface outputs.
 
I checked all the channel, sends, & bus outputs. All good.
 
I have 40 tracks, 6 folders, & 7 buses.
 
If I solo each folder set I get sound.
 
If I solo each bus I get sound.
 
If I mute the Master bus there's no sound.
 
This happens only in one project. I opened a newer version of the same project at the same time & the newer one does not do this.
 
Project file corruption?
 
EDIT: This is the 54th version of this project, opened and saved using different versions of Sonar. Each time I make a major change I "Save as" to a new file.